The present invention relates to novel alumina-based extrudates. It also relates to processes for preparing these extrudates and to their use as catalyst supports, in particular for hydrotreatment.
Hydrotreatment of petroleum cuts is a purification operation carried out in hydrogen which encompasses several types of treatment, in particular: hydrodesulphurisation, hydrodemetallisation, hydrodenitrogenation, Conradson Carbon number hydroreduction (HDCC), asphaltene hydrogenation (HDAS) and aromatic hydrogenation (HDAr).
Hydrodemetallisation can eliminate metals, in particular nickel and vanadium, contained in the heavy petroleum fractions from vacuum distillation of the first refinery stages.
Hydrodesulphurisation can eliminate sulphur-containing compounds such as RSH or R2S, R being an alkyl radical. It is primarily used on light petroleum fractions or those with a low metals (Ni and V) content.
When hydrotreating heavy petroleum cuts, it is important for the catalyst used to be equilibrated for hydrodemetallisation and hydrodesulphurisation, i.e., that it has the same efficacy for the two activities.
Further, the catalyst must have a good retention capacity for hydrodemetallisation. In contrast to hydrodesulphurisation where the reaction products remain in the reaction medium, the products resulting from hydrodemetallisation remain absorbed on the catalyst. It is thus important that the catalyst can absorb as much as possible of the metallic derivatives from hydrodemetallisation as once saturated with metallic derivatives, it has to be replaced.
During catalytic hydrodemetallisation, metal elimination encounters a certain number of catalytic problems, in particular as regards the catalytic support.
A first problem arises from the fact that the metallic compounds are high molecular weight compounds, such as asphaltenes and resins, with a high degree of steric hindrance resulting in diffusional limitations in the catalyst""s pore network.
A second problem arises since the metals, of more exactly the metal sulphides, from the decomposition of the metallic compounds accumulate during the operation, gradually reducing the pore diameter.
A third problem arises because the metallic compounds of the asphaltene type tend to dehydrogenate then to polycondense under the operating conditions to give rise to products which are insoluble in the hydrocarbon medium and to coke which remains fixed on the catalyst thus gradually reducing its activity.
A heavy cut hydrotreatment catalyst must thus be composed of a catalytic support with a porosity profile which is particularly adapted for the specific diffusional constraints encountered in hydrotreatment, in particular hydrodemetallisation.
The catalysts are normally composed of a support on which metallic oxides, such as cobalt, nickel or molybdenum oxides are deposited. The support is generally alumina-based, its role being to disperse the active phase and to provide a texture which is suitable for good capture of the metallic impurities, avoiding the problems defined above.
Prior art alumina-based supports fall into two categories.
The first category consists of alumina extrudates prepared from alumina gel. Such alumina gel extrudates represent the majority of catalyst supports for hydrotreatment, but they suffer from a number of disadvantages.
Firstly, their porosity is particularly adapted for hydrodesulphurising and hydrotreating light hydrocarbon-containing cuts, but not for other types of treatment.
Then, although such extrudates are equilibrated as regards their hydrodemetallisation/hydrodesulphurisation ratio, their hydrodemetallisation retention capacity is low, in general at most 30%, so they are rapidly saturated and have to be replaced.
Further, alumina gel is expensive, meaning that the supports which are produced are expensive.
Finally, the process for preparing the alumina gel is particularly polluting in contrast to that of the alumina produced by rapid dehydration of hydrargillite, known as flash alumina.
Alumina beads prepared from alumina from the rapid dehydration of hydrargillite are also used for hydrotreatment, at a lower cost. Such beads, however, suffer from a disadvantage: the bead diameter has to be more than 2 mm for a satisfactory cost price, meaning that the metals cannot be introduced to the core of the beads. The catalytic phase with which the bead core is impregnated is thus not used.
Smaller flash alumina extrudates would not have this disadvantage, but currently there is no preparation process which would produce such flash alumina extrudates with a porosity suitable for hydrotreatment.
One aim of the present invention is to provide cheap alumina-based supports for hydrotreatment.
Another aim of the present invention is to provide alumina extrudates produced by rapid dehydration of hydrargillite with characteristics which are adapted for their use as supports for hydrotreatment catalysts.
A still further aim of the present invention is to provide processes for preparing these alumina extrudates.
With these aims in mind, in a first aspect, the invention provides alumina extrudates produced by a preparation process starting from an alumina produced by the rapid dehydration of hydrargillite with a total pore volume of at least 0.6 cm3/g, an average mesoporous diameter in the range 140 to 360 xc3x85, and an alumina content produced by boehmite decomposition in the range 5% to 70% by weight.
The invention also concerns a first process for forming an alumina from a starting alumina produced by the rapid dehydration of hydrargillite, comprising the following steps:
a1 starting with an alumina produced by rapid dehydration of hydrargillite;
b1 rehydrating the starting alumina;
c1 mixing the rehydrated alumina in the presence of an emulsion of at least one hydrocarbon in water;
d1 extruding the alumina-based paste obtained from step c1;
e1 drying and calcining the extrudates;
f1 carrying out a hydrothermal acid treatment in a confined atmosphere on the extrudates from step e1;
g1 optionally drying, then calcining the extrudates from step f1.
The invention also concerns a second process for forming an alumina from a starting alumina produced by the rapid dehydration of hydrargillite, comprising the following steps:
a2 starting from a starting alumina produced by rapid dehydration of hydrargillite;
b2 forming the alumina into beads in the presence of a pore-forming agent;
c2 ageing the alumina beads obtained;
d2 mixing the beads from step c2 to obtain a paste which is extruded;
e2 drying and calcining the extrudates obtained;
f2 carrying out a hydrothermal acid treatment in a confined atmosphere on the extrudates obtained from step e2;
g2 optionally drying, then calcining the extrudates from step f2.
The invention also concerns a third process for forming an alumina from a starting alumina produced by the rapid dehydration of hydrargillite, comprising the following steps:
a3 starting from an alumina produced by rapid dehydration of hydrargillite;
b3 rehydrating the starting alumina;
c3 mixing the rehydrated alumina with a pseudo-boehmite gel, the gel being present in an amount in the range 1% to 30% by weight with respect to the rehydrated alumina and the gel;
d3 extruding the alumina-based paste obtained from step c3;
e3 drying and calcining the extrudates obtained;
f3 carrying out a hydrothermal acid treatment in a confined atmosphere on the extrudates obtained from step e3;
g3 optionally drying, then calcining the extrudates from step f3.
Finally, the invention concerns the use of this alumina as a catalyst or catalyst support.
FIGS. 1 and 2 are photographs of sections of an extrudate of the invention produced by transmission electron microscopy.
In hydrotreatment, the extrudates of the invention have the advantage of being equilibrated in hydrodemetallisation and hydrodesulphurisation and of having a good retention capacity for hydrodemetallisation.
Other advantages of the invention will become apparent from the following more detailed description.
Firstly, the invention concerns alumina extrudates produced by a preparation process starting from an alumina produced by the rapid dehydration of hydrargillite with a total pore volume of at least 0.6 cm3/g, an average mesoporous diameter in the range 140 to 360 xc3x85, and an alumina content produced by boehmite decomposition in the range 5% to 70% by weight.
The term xe2x80x9cproduced by boehmite decompositionxe2x80x9d means that during the extrudate preparation process, boehmite type alumina is developed to the point of representing 5% to 70% by weight of the total alumina, then is decomposed. This amount of alumina produced by boehmite decomposition is measured by X ray diffraction before decomposition of said boehmite.
The alumina extrudates of the invention generally and preferably have a total pore volume (TPV) of at least 0.6 cm3/g, preferably at least 0.65 cm3/g.
The TPV is measured as follows: the grain density and absolute density are determined: the grain densities (Dg) and absolute densities (Da) are measured using a mercury and a helium picnometry method respectively, then the TPV is given by the formula:   TPV  =            1              D        g              -          1              D        a            
The average mesoporous diameter of the extrudates of the invention is also in the range 150 to 360 xc3x85. The average mesoporous diameter for the given extrudates is measured using a graph of the pore distribution of said extrudates. It is the diameter for which the associated volume on the graph is:       V          100      ⁢      Å        +                    V                  60          ⁢          Å                    -              V                  1000          ⁢          Å                      2  
where V1000 xc3x85 represents the volume created by pores with a diameter of over 1000 xc3x85, or the macroporous volume;
V60 xc3x85 represents the volume created by pores with a diameter of over 60 xc3x85;
V60 xc3x85-V1000 xc3x85 represents the mesoporous volume, i.e., the volume created by pores with a diameter in the range 60 xc3x85 to 1000 xc3x85, i.e., the volume created by all the pores with a size in the range 60 xc3x85 to 1000 xc3x85.
These volumes are measured using the mercury penetration technique in which the Kelvin law is applied which defines a relationship between the pressure, the diameter of the smallest pore into which the diameter penetrates at that pressure, the wetting angle and the surface tension in the following formula:
Ø=(4t cos xcex8)/P
where
Ø represents the pore diameter;
t represents the surface tension;
xcex8 represents the angle of contact; and
P represents the pressure.
The mesoporous volume (V60 xc3x85-V1000 xc3x85) of the extrudates of the invention is preferably at least 0.3 cm3/g, more preferably at least 0.5 cm3/g.
The macroporous volume (V1000 xc3x85) of the extrudates of the invention is preferably at most 0.5 cm3/g. In a variation, the macroporous volume (V1000 xc3x85) is at most 0.3 cm3/g, more preferably at most 0.1 cm3/g and still more preferably at most 0.08 cm3/g.
These extrudates normally have a microporous volume (V0-60 xc3x85) of at most 0.55 cm3/g, preferably at most 0.2 cm3/g. The microporous volume represents the volume created by pores with a diameter of less than 60 xc3x85.
Such a pore distribution which minimises the proportion of pores of less than 60 xc3x85 and those of more than 1000 xc3x85 while increasing the proportion of mesopores (with a diameter in the range 60 xc3x85 to 1000 xc3x85) is particularly adapted to the diffusional constraints of hydrotreating heavy hydrocarbon cuts.
In a variation, the pore distribution over the pore diameter range from 60 xc3x85 to 1000 xc3x85 (mesopores) is extremely narrow at around 140 xc3x85, i.e., in this range the diameter of the majority of pores is preferably in the range 60 xc3x85 to 500 xc3x85, more preferably in the range 80 xc3x85 to 200 xc3x85.
The specific surface area (SSA) of the extrudates of the invention is generally at least 120 m2/g, preferably at least 150 m2/g. This surface area is a BET surface area. The term xe2x80x9cBET surface areaxe2x80x9d means the specific surface area determined by nitrogen adsorption in accordance with the standard ASTM D 3663-78 established using the BRUNAUER-EMMETT-TELLER method described in xe2x80x9cThe Journal of the American Societyxe2x80x9d 60, 309 (1938).
Preferably, the diameter of the extrudates of the invention is in the range 0.3 to 10 mm, more preferably in the range 0.8 to 3.2 mm, and the length is in the range 1 mm to 20 mm, preferably in the range 1 to 10 mm.
The average crushing strength (ACS) of these extrudates is generally at least 0.7 kg/mm for 1.6 mm extrudates, preferably at least 1, and the crush strength (CS) is at least 1 MPa.
The method of measuring the average crushing strength (ACS) consists of measuring the type of maximum compression which an extrudate can support before it fails, when the product is placed between two planes being displaced at a constant speed of 5 cm/min.
Compression is applied perpendicular to one of the extrudate generatrices, and the average crushing strength is expressed as the ratio of the force to the length of the generatrix of the extrudate.
The method used to measure the crush strength (CS) consists of subjecting a certain quantity of extrudates to an increasing pressure over a sieve and recovering the fines resulting from crushing the extrudates. The crush strength corresponds to the force exerted to obtain fines representing 0.5% of the weight of the extrudates under test.
On the microscopic scale, the alumina of the invention is partially in the form of packs of flakes and partially in the form of needles, the needles being dispersed both around the packs of flakes and between the flakes.
In general, the length and breadth of the flakes varies between 1 and 5 xcexcm with a thickness of the order of 10 nm. They can be packed in groups forming a thickness of the order of 0.1 to 0.5 xcexcm, the groups possibly being separated from each other by a thickness of the order of 0.05 to 0.1 xcexcm.
The needle length can be in the range 0.05 to 0.5 xcexcm; their cross section is of the order of 10 to 20 nm. These dimensions are given by measuring the extrudates in electron microscope photographs. The needles between the flakes are generally smaller than the needles surrounding the flakes.
The alumina flakes principally comprise "khgr" alumina and xcex7 alumina and the needles are xcex3 alumina.
The flake structure is characteristic of the hydrargillite lineage of alumina, which means that before activation by calcining these extrudates have the same structure, the flakes being hydrargillite in nature. On calcining, this alumina in its hydrargillite form is principally transformed into dehydrated "khgr" and xcex7 aluminas
In contrast, the needle structure is characteristic of a boehmite lineage, meaning that before activation by calcining, these extrudates have the same structure, this alumina being in the form of boehmite. Calcining transforms this boehmite alumina into dehydrated xcex3 alumina.
The extrudates of the invention are thus obtained by calcining, the extrudates being constituted by hydrargillite alumina-based flakes prior to calcining, the flakes being surrounded at their periphery by boehmite alumina-based needles, such needles also being found around the flakes.
The invention also concerns a first process for forming such an alumina from a starting alumina produced by rapid dehydration of hydrargillite, characterized in that it comprises the following steps:
a1 starting with an alumina produced by rapid dehydration of hydrargillite;
b1 rehydrating the starting alumina;
c1 mixing the rehydrated alumina in the presence of an emulsion of at least one hydrocarbon in water;
d1 extruding the alumina-based paste obtained from step c1;
e1 drying and calcining the extrudates;
f1 carrying out a hydrothermal acid treatment in a confined atmosphere on the extrudates from step e1;
g1 optionally drying, then calcining the extrudates from step f1.
The forming process of the invention is suitable for a specific starting alumina produced by rapid dehydration of Bayer hydrate (hydrargillite) which is an industrially available aluminium hydroxide and extremely cheap.
Such an alumina is in particular obtained by rapid dehydration of hydrargillite using a hot gas stream, the temperature of the gas entering the apparatus generally being between about 400xc2x0 C. and 1200xc2x0 C., the contact time between the alumina and the hot gases generally being in the range from a fraction of a second to 4-5 seconds; such a process for preparing an alumina powder has been described in French patent FR-A-1 108 011.
The alumina obtained can be used as it is or before undergoing step b1, it can be treated to eliminate the alkalis present: a Na2O content of less than 0.5% by weight is preferable.
In step b1, this starting alumina, which is in the form of a powder, is rehydrated. Rehydration transforms alumina to a rehydrated alumina which is partially in the form of a boehmite type alumina with a particular texture.
The starting alumina is preferably re-hydrated so that the boehmite type alumina content is at least 3% by weight, more preferably at most 40% by weight, in particular in the range 10% to 35%.
This rehydration can be carried out using any means known to the skilled person. Preferably, the starting alumina is rehydrated by suspending in water in the presence of an Al3+ cation complexing agent.
This complexing agent can be selected from the group formed by:
acids comprising at least one carboxylate group and at least two hydroxyl and/or amine groups, or comprising at least two carboxylate groups and at least one hydroxyl and/or amine group, and salts thereof,
organic phosphoric acids with the following formulae (I), (II) and (III):
9/15
where n and m are whole numbers in the range 1 to 6, p is a whole number in the range 0 to 5, R1, R2 and R3, which may be identical or different, represent a hydrogen atom, an aralkyl, aryl, alkyl group or a hydroxyl or amine group,
hydroxyquinolines or derivatives with the following formula (IV):
9/36
where R represents a hydrogen atom, a hydrocarbon radical containing 1 to 20 carbon atoms or a halogen atom.
This complexing agent is preferably a carboxylic acid containing 2 to 15 carbon atoms. It can be selected from:
oxalic acid,
hydroxypolycarboxylic acids, more particularly:
hydroxydi- or hydroxytri-carboxylic acids such as malic acid, citric acid or tartronic acid;
(polyhydroxy)monocarboxylic acids such as glucoheptonic acid and gluconic acid;
poly(hydroxycarboxylic) acids such as tartaric acid.
The salts of these acids are also suitablexe2x80x94, such as alkali metal salts, in particular sodium salts, and ammonium salts.
Regarding the complexing agents with formulae (I) to (III), an organic, preferably substituted or non substituted aliphatic phosphoric acid is used. This contains 1 to 15 carbon atoms, preferably 1 to 10.
Examples of such compounds which can be cited are:
methylene aminotrophosphonate, methylene ethylenediaminotetraphosphate, methylene triethylenetetraaminohexaphosphonate, methylene tetraethylenepentaaminoheptaphosphonate, methylene pentaethylenehexaaminooctaphosphonate;
methylene, 1,1xe2x80x2-ethylene, 1,2-ethylene; 1,1xe2x80x2-propylene, 1,3-propylene, 1,6-hexa methylene diphosphonate; 2,4-dihydroxypentamethylene-2,4-diphosphonate, 2,5-dihydroxyhexamethylene-2,5-diphosphonate; 2,3-dihydroxybutylene-2,3-diphosphonate; 1-hydroxybenzyl-1,1xe2x80x2-diphosphonate; 1-aminoethylene- 1,1xe2x80x2-diphosphonate; hydroxymethylene diphosphonate; 1-hydroxyethylene-1,1xe2x80x2-diphosphonate; 1-hydroxypropylene-1,1xe2x80x2-diphosphonate; 1-hydroxybutylene-1,1xe2x80x2-diphosphonate; 1-hydroxyhexamethylene-1,1xe2x80x2-diphosphonate.
Regarding the complexing agents with formula (IV), a compound with formula (IV) is preferably used wherein R is a hydrogen atom or a linear or branched, saturated or unsaturated aliphatic group preferably containing 5 to 20 carbon atoms.
Preferably, an oxine (hydroxy-8-quinoline) is used or a hydroxy-8-quinoline with formula (IV) where R is always a hydrogen atom, with the exception of the 7 position on the ring where it represents an alkyl group with formula CmH2m+1 where m is in the range 5 to 20.
The complexing agent defined is used in the solid form or preferably in the form of an aqueous solution with a concentration which preferably is in the range 0.01 to 1 mole/liter.
The quantity of complexing agent introduced with respect to the quantity of alumina is determined such that it preferably represents 0.01% to 5% by weight of the alumina, more preferably 0.05% to 1%.
The preferred complexing agent is citric acid. It can be used in an amount in the range 0.1% to 1% by weight with respect to the alumina.
The suspension of alumina in the presence of the Al3xe2x88x92 complexing agent is generally carried out at a concentration in the range 200 to 700 g/l of alumina in water, preferably 500 g/l.
The temperature during step b1 can be between 50xc2x0 C. and 100xc2x0 C. for a period in the range from a few hours to 72 hours.
The suspended alumina is then filtered and a cake is recovered which can be washed with water.
In step c1, the rehydrated alumina is then mixed in the presence of an emulsion of at least one hydrocarbon in water.
Optionally, between steps b1 and c1, the rehydrated alumina from step b1 can be dried. This drying must be carried out under conditions allowing the water impregnating the rehydrated alumina to be eliminated but without eliminating the water bonded to the alumina, and therefore the drying temperature is at most 250xc2x0 C. As an example, the cake can be dried at a temperature in the range 60xc2x0 C. to 150xc2x0 C. Preferably, the cake is dried so that the dried alumina powder has a loss on ignition, measured by calcining at 1000xc2x0 C., in the range 20% to 40%.
During step c1, the rehydrated alumina is mixed with an emulsion of at least one hydrocarbon in water. This emulsion is obtained by mixing the hydrocarbon and a surfactant in water.
This hydrocarbon can be selected from those containing 10 to 15 carbon atoms and a boiling point in the range 185xc2x0 C. to 200xc2x0 C. such as lamp oil, paraffin oil or any other alkane cut, and mixtures thereof.
The aqueous phase/hydrocarbon phase ratio of the emulsion can be at least 1.
The quantity of hydrocarbon, in emulsion in water, mixed with the rehydrated alumina is generally at least 5% by weight with respect to the weight of rehydrated alumina, preferably at least 10% and at most 50%.
In a preferred implementation of the process of the invention, the petroleum-in-water emulsion comprises a non ionic surfactant.
In a particularly preferred mode, it is a non ionic surfactant with ethylene oxide (EO)-propylene oxide (PO) double blocks, in particular those containing at least 10 PO motifs.
The surfactant is preferably selected from those for which the hydrocarbon-in-water emulsion is stable in an acidic medium. To verify this property, an emulsion as defined above is prepared by mixing and stirring water, a hydrocarbon and the surfactant to be tested, then adding an acid. If the three phases: water/surfactant/hydrocarbon are observed to settle out by separating, the emulsion is not stable in an acidic medium.
To this end, the following commercially available surfactants can be used: Galoryl EM 10(copyright) from the Compagnie Francaise des Produits Industriels, and Soprophor SC138(copyright) sold by Rhxc3x4ne-Poulenc Geronazzo.
In general, the amount of surfactant in the emulsion is at least 3% by weight with respect to the hydrocarbon, preferably in the range 5% to 15%.
The emulsion is generally prepared by bringing the different constituents (water, hydrocarbons, surfactants) into contact then stirring.
In a preferred implementation of the invention, the rehydrated alumina and the emulsion are mixed in an acidic medium, in particular in the presence of nitric acid. The acid can be introduced via the emulsion.
Mixing can be carried out using any mode known to the skilled person, in particular using a Z blade mixer or a twin screw mixer.
Water can be added to adjust the viscosity of the paste to be extruded.
When mixing is carried out in an acidic medium, the alumina-based paste is preferably neutralised between steps c1 and b1. This neutralisation can be carried out using an ammonia type base. The neutralising agent is normally introduced when mixing is complete.
During step d1, the paste from the mixing step is extruded through a die, for example using a piston or a twin extrusion screw. This extrusion step can be carried out using any method which is known to the skilled person.
In step e1, after forming, the alumina extrudates are dried and calcined. The drying temperature can be in the range 100xc2x0 C. to 200xc2x0 C., and the calcining temperature in the range 200xc2x0 C. to 700xc2x0 C.
The extrudates from step e1 then undergo acid hydrothermal treatment in a confined atmosphere (step f1).
The term xe2x80x9chydrothermal treatment in a confined atmospherexe2x80x9d means treatment by passage through an autoclave in the presence of water at a temperature higher than ambient temperature.
During this hydrothermal treatment, the formed alumina can be treated in different ways. Thus the alumina can be impregnated with acid prior to its passage through the autoclave, alumina autoclaving being carried out either in the vapour phase or in the liquid phase; this vapour or liquid phase of the autoclave may or may not be acidic. This impregnation prior to autoclaving can be carried out dry or by immersing the alumina in an aqueous acidic solution. The term xe2x80x9cdry impregnationxe2x80x9d means bringing the alumina into contact with a volume of solution which is less than or equal to the total pore volume of the treated alumina. Impregnation is preferably dry impregnation.
It is also possible to treat the agglomerates without prior impregnation with an acid, the acidity in this case being provided by the aqueous liquid of the autoclave.
The hydrothermal treatment is normally carried out in the presence of an aqueous acidic solution constituted by a mixture of at least one acid which can dissolve at least a portion of the alumina of the extrudates and at least one compound which provides an anion which can combine with the aluminium ions in solution. The term xe2x80x9cacid which can dissolve at least a portion of the alumina of the extrudatesxe2x80x9d means any acid which, brought into contact with the alumina extrudates, dissolves at least a portion of the aluminium ions. The acid preferably dissolves at least 0.5% by weight of the alumina of the alumina extrudates.
This acid is preferably selected from strong acids such as: nitric acid, hydrochloric acid, perchloric acid, sulphuric acid, or a weak acid used at a concentration such that the pH of the aqueous solution is less than 4, such as acetic acid, or a mixture of these acids.
The term xe2x80x9ccompound providing an anion which can combine with the aluminium ions in solutionxe2x80x9d means any compound which can liberate an anion Axe2x88x92n in solution which can form, with the Al3+ cations, products in which the atomic ratio nA/Alxe2x89xa73.
A particular case of these salts can be illustrated by basic salts with general formula Al2(OH)xAy in which 0 less than x less than 6; n y less than 6; n represents the number of charges on anion A. Compounds which can liberate anions in solution can preferably liberate anions selected from the group formed by nitrate, chloride, sulphate, perchlorate, chloroacetate, dichloroacetate, trichloroacetate, bromoacetate, dibromoacetate and anions with general formula:
13/31
where R represents a radical selected from the group formed by H, CH3, C2H5, CH3CH2CH2, (CH3)2CH. The anions can be liberated in solution as the anion Axe2x88x92n, either directly by dissociation, or indirectly, for example by hydrolysis. The compounds can in particular be selected from groups comprising: mineral or organic acids, anhydrides, organic or mineral salts, and esters. The salts include alkaline or alkaline-earth salts which are soluble in aqueous media, such as sodium, potassium, magnesium or calcium salts, ammonium salts, aluminium salts, or rare earth salts.
In a preferred mode, the hydrothermal treatment is carried out in the presence of a mixture of nitric acid and acetic acid. The respective quantities of these two acids are generally in the range 3% to 15% by weight.
The autoclave is preferably a rotating basket autoclave such as that defined in European patent application EP-A-0 387 109.
The temperature during autoclaving can be in the range 150xc2x0 C. to 250xc2x0 C. for a period in the range 30 minutes to 3 hours.
The treatment can be carried out under saturation vapour pressure or under a partial pressure of water vapour of at least 70% of the pressure of the saturation vapour pressure corresponding to the treatment temperature.
This hydrothermal treatment in a confined atmosphere leads to transformation of at least a portion of the alumina into the boehmite form.
The hydrothermal treatment must be carried out such that at the end of step f1, the alumina has a boehmite type alumina content of at most 70% by weight, preferably at least 5% by weight. The skilled person will know how to adjust the autoclave parameters to obtain such contents.
After the hydrothermal treatment step f1, the extrudates are optionally dried, then calcined (step g1). The drying temperature can be in the range 100xc2x0 C. to 200xc2x0 C. and the calcining temperature is in the range 200xc2x0 C. to 700xc2x0 C.
The invention also concerns a second process for forming an alumina as described above, from a starting alumina produced by the rapid dehydration of hydrargillite as described above, characterized in that it comprises the following steps:
a2 starting from a starting alumina produced by rapid dehydration of hydrargillite;
b2 forming the alumina into beads in the presence of a pore-forming agent;
c2 ageing the alumina beads obtained;
d2 mixing the beads from step c2 to obtain a paste which is extruded;
e2 drying and calcining the extrudates obtained;
f2 carrying out a hydrothermal acid treatment in a confined atmosphere on the extrudates obtained from step e2;
g2 optionally drying, then calcining the extrudates from step f2.
This second forming process of the invention is also suitable for a specific starting alumina produced by the rapid dehydration of Bayer hydrate.
During step b2, the alumina is formed into beads.
This forming step can be carried out using any technique which is known to the skilled person. It is carried out directly on the alumina powder using a rotary technique. The term xe2x80x9crotary techniquexe2x80x9d means any apparatus in which agglomeration is carried out by contact and rotating the product to be granulated on itself. Apparatus of this type which can be cited include the rotary bowl granulator and the rotating drum.
This forming is carried out in the presence of a pore-forming agent. Examples of pore-forming compounds which can be cited include wood flour, wood charcoal, sulphur, tars, plastics materials or emulsions of plastic materials such as polyvinyl chloride, polyvinyl alcohols, naphthalene or the like, and in general any organic compound which can be eliminated by calcining. The quantity of pore-forming compounds added is not critical, nor is their size. In general, the quantity of pore-forming agents is in the range 5% to 30% by weight with respect to the alumina.
The size of the beads obtained is not critical. In general it is in the range 1 to 5 mm.
The beads from step b2 then undergo ageing (step c2). Ageing can be carried out by maintaining the alumina beads in an atmosphere with a controlled humidity. The temperature can be in the range 30xc2x0 C. to 100xc2x0 C., preferably in the range 80xc2x0 C. to 100xc2x0 C. The ageing period can be from a few hours to a few tens of hours, preferably in the range 6 to 24 hours.
During this ageing step, the alumina rehydrates to boehmite (Al2O3,H2O) and bayerite (Al2O3,3H2O).
The ageing conditions must be selected to allow the crystalline boehmite phase of the alumina to develop. At the end of step c2, the amount of boehmite type alumina is preferably in the range 5% to 40%, more preferably 20% to 30% with respect to the weight of alumina.
One practical ageing mode consists of injecting live steam onto the alumina beads.
During step d2, the aged alumina beads are mixed in the presence of water and acid so as to break them up and obtain an extrudable homogeneous paste. The acid used can be a strong acid (for example nitric acid) or acetic acid, formic acid, etc.
The quantity of acid with respect to the alumina is generally in the range 0.5% to 15% by weight.
Mixing can be carried out using any method known to the skilled person, in particular using a Z blade mixer or a twin screw mixer.
This paste is then extruded through a die, for example using a piston or a continuous twin screw or single screw extruder. This extrusion step can be carried out using any method which is known to the skilled person. The viscosity of the paste to be extruded is adjusted by the quantity of water added during mixing.
Then in a step e2, after extrusion, the extrudates are dried and calcined. As above, the drying temperature can be in the range 100xc2x0 C. to 200xc2x0 C., the calcining temperature is in the range 200xc2x0 C. to 800xc2x0 C.
The extrudates from step e2 then undergo acid hydrothermal treatment in a confined atmosphere (step f2). The hydrotreatment conditions are identical to those defined for the first preparation process described above.
After the hydrothermal treatment step f2, the extrudates are optionally dried, then calcined (step g2). As was the case for the first process, the drying temperature can be in the range 100xc2x0 C. to 200xc2x0 C., and the calcining temperature in the range 200xc2x0 C. to 700xc2x0 C.
The invention also concerns a third process for forming an alumina as described above from a starting alumina produced by the rapid dehydration of hydrargillite as described above, characterized in that it comprises the following steps:
a3 starting from an alumina produced by rapid dehydration of hydrargillite;
b3 rehydrating the starting alumina;
c3 mixing the rehydrated alumina with a pseudo-boehmite gel, the gel being present in an amount in the range 1% to 30% by weight with respect to the rehydrated alumina and the gel;
d3 extruding the alumina-based paste obtained from step c3;
e3 drying and calcining the extrudates obtained;
f3 carrying out a hydrothermal acid treatment in a confined atmosphere on the extrudates obtained from step e3;
g3 optionally drying, then calcining the extrudates from step f3.
This process uses steps identical to steps a1, b1, d1, e1, f1 and g1 of the first process described above.
However, in step c3, the rehydrated alumina from step b3 is not mixed with a hydrocarbon emulsion but with a pseudo-boehmite gel in an amount in the range 1% to 30% by weight with respect to the rehydrated alumina and to the gel, preferably in the range 5% to 20% by weight.
Such a pseudo-boehmite gel can be obtained by precipitating aluminium salts such as aluminium chloride, aluminium sulphate, aluminium nitrate, aluminium acetate with a base or by hydrolysis of aluminium alkoxides such as aluminium triethoxide.
Mixing can be carried out by any method which is known to the skilled person, in particular using a Z blade mixer or a twin screw mixer.
Water can be used to adjust the viscosity of the paste to be extruded.
Finally, the invention concerns the use of the alumina extrudates described above or from the above processes as catalysts, catalyst supports or adsorbents.
The right hand side of FIG. 1 shows a pack of flakes surrounded by needles. This pack is surrounded by needles on its left hand side. Needles can also be seen inside the pack of flakes. The scale of the photograph is such that 1 cm on the photo represents 263 nm.
FIG. 2 shows a flake separated from the other flakes by needles. The scale of the photograph is such that 1 cm on the photo represents 79 nm.